Ask Dr. Dave

In his August 2008 column, Dr. Dave discusses the best way to seal porous metal castings and powdered metal components, and the difference between one- and two-component structural adhesives.

Question: What is the best way
to seal porous metal castings and powdered metal components?

Answer: The modern way of doing this is to use
either heat-cured or room-temperature-cured, methacrylate-based impregnation
sealants. These are low-viscosity sealants (typically less than 30cps) that are
impregnated into pores by submersing the parts in the sealant, applying a
vacuum to remove the air from the pores and then restoring atmospheric
pressure, whereby the sealant penetrates into the evacuated pores. Following
the impregnation, the sealant cures to a polymeric resin and seals the
porosity. Most sealants are cured using heat via an oven or hot water at
85-95°C. However, an alternative system uses the so-called “anaerobic” cure
system where the resin cures inside the pores at room temperature. Anaerobic
sealants require exposure to atmospheric oxygen via an aeration grid between
vacuum cycles; cooling of the sealant to maintain stability; and adjustment of
the reactivity of the resin by chemical additions. The equipment for heat-cured
sealants is much simpler, with no cooling, aeration or chemical additions, and
this system is usually the most cost-effective solution for most sealing
problems. However, there are instances where anaerobic sealants are most
effective, such as in heat-sensitive electrical components and highly porous
powdered metal components, where a heat cure sometimes causes “bleed-out” of
sealant due to the differences in thermal expansion coefficients of the sealant
and the metal.

Question:
As a manufacturing engineer in the auto industry, I use a lot of structural
adhesives. However, my suppliers always want me to use two-component structural
adhesives that require complicated mixing and application equipment. Why can’t
I use one-component adhesives?

Answer: Two-component structural adhesives such as
epoxies or urethanes are used for fast cure speeds, good gap-filling ability,
and high performance. Unfortunately, the trade-off with two-components is the
need for mixing equipment. The only one-component alternatives are heat-cured
epoxies, reactive hot-melt urethanes or toughened cyanoacrylates. The epoxies
require heating for cure cycles ranging from 30 minutes to several hours,
whereas the reactive hot-melt urethanes are applied in molten form and fixture
almost instantly but require hours or days to fully cure through reaction with
atmospheric moisture. Toughened cyanoacrylates are quite different from common
cyanoacrylate-based super glues and are true structural adhesives. However,
they suffer from limited ability to fill gaps and very high costs, making them
suitable only for small-area bonding.

Ask Dr. Dave is written by Dave Dunn, Ph.D., of F.L.D.Enterprises, a technical consultancy and full-service industrial market-research firm specializing in the adhesives, sealants, specialty rubbers and plastics fields. Dave is a former vice president and director of Loctite Corp. and has spent many years in troubleshooting adhesive and sealant problems. Questions for publication should be directed to him at 242 Trails End, Aurora OH 44202; phone (440) 477-5164; fax (330) 247-1690; or email DrDave242@windstream.net .

Products

The Handbook of Sealant Technology provides an in-depth examination of sealants, reviewing their historical developments and fundamentals, adhesion theories and properties, and today’s wide range of applications.